What is backpressure in Node.js streams?

Direct answer

Backpressure slows a fast producer when the consumer cannot keep up — writable.write() returns false, pause the readable, resume on 'drain'; pipeline handles this automatically; ignoring it causes memory spikes and OOM.

Without backpressure, a fast disk read flooding a slow network response buffers unbounded chunks in RAM — production APIs hit OOM under load.

Why interviewers ask

write returns false — stop writing until writable emits 'drain'.
readable.pause() — manual backpressure when not using pipe/pipeline.
highWaterMark — default 16KB (object mode: 16 objects); tune for throughput vs memory.
pipeline() — wires pause/resume across the chain correctly.

backpressure.mjs

function writeWithBackpressure(readable, writable) {
  readable.on("data", (chunk) => {
    const ok = writable.write(chunk);
    if (!ok) {
      readable.pause();
      writable.once("drain", () => readable.resume());
    }
  });
  readable.on("end", () => writable.end());
}

// Prefer:
// import { pipeline } from "node:stream/promises";
// await pipeline(readable, writable);